Sustainable diets have been forwarded as initiatives to mitigate the impact of current food consumption towards health and the environment. For instance, adopting a flexitarian diet and reducing the consumption of animal-sourced food (ASF) are said to help limit the rate of food system greenhouse gas emissions. However, much remains to be explored on how individuals associate themselves with alternative diets. In filling this gap, self-declared omnivores and flexitarians from ten European countries (N = 5870) were surveyed to determine how they identify with a particular diet, considering their food preferences and consumption frequency of ASF and ASF substitutes. The analysis of the data revealed that flexitarians seemed to consume more plant-based alternatives than omnivores, yet their consumption of ASF remained comparable. A series of segmentation analyses likewise revealed that some individuals (N = 563, 34.2%) reporting as flexitarians seemed to behave similarly to conventional omnivores and conversely, some omnivores (N = 1202, 28.5%) behaved similarly to conventional flexitarians in terms of food consumption frequencies. Moreover, significant associations with consuming plant-based food were found when considering the current readiness level of the respondent to transform diets. Taken together, the findings of this study show that identifying with a particular dietary lifestyle is contingent on how individuals set personal thresholds to qualify for a particular diet, thereby providing implications to the way nutritional guidelines frame and define recommendations for dietary intake.